Rare earth metal-based sintered magnets such as R—Fe—B based sintered magnets represented by an Nd—Fe—B based sintered magnet are widely used nowadays in various fields because they use materials available from abundant and inexpensive resources, and possess high magnetic characteristics; however, since they contain highly reactive rare-earth metal, R, also characteristic is that they are apt to be oxidized and corroded in the ambient. Accordingly, in practical use, a corrosion resistant film such as a metal film or a resin film is generally formed on the surface of the rare earth metal-based sintered magnet. However, in an embodiment in which the magnet is embedded and used inside a component, such as in an IPM (Interior Permanent Magnet) motor, the corrosion resistant film above need not necessarily be formed on the surface of the magnet. Still, as a matter of course, corrosion resistance of a magnet must be ensured for the time period after the production to the embedding of the magnet into the components. Thus, as a method for ensuring corrosion resistance of the rare earth metal-based sintered magnet for the time period above, there has been proposed a method of modifying the surface of the magnet by carrying out a heat treatment under an oxidative atmosphere, and this method has attracted attention as an easy technique for improving corrosion resistance, which is enough to achieve the objective above.
The oxidative atmosphere necessary for carrying out the oxidative heat treatment for a surface modification of the rare earth metal-based sintered magnet may be formed by using oxygen (reference can be made to Patent Literature 1 or Patent Literature 2) or water vapor. For instance, Patent Literatures 3 to 6 describe methods for forming an oxidative atmosphere, which comprises by using water vapor alone, or by using the combination of water vapor and oxygen.
Patent Literature 1: U.S. Pat. No. 2,844,269
Patent Literature 2: JP-A-2002-57052
Patent Literature 3: JP-A-2006-156853
Patent Literature 4: JP-A-2006-210864
Patent Literature 5: JP-A-2007-103523
Patent Literature 6: JP-A-2007-207936